Exim does not have any specific code for transporting mail via UUCP, nor does
it support UUCP bang path addresses. However, if domain addressing is being
used, Exim can be interfaced to UUCP fairly simply. Here is a configuration
fragment for sending certain domains to UUCP, taken from a real installation:

In a complete configuration file, the transport would be inserted
among the other transports, and the router probably defined as the
first router. The file /usr/exim/uucphosts
contains entries like this:

darksite.example.com: darksite.UUCP

which is interpreted to mean, “Send mail addressed to the domain
darksite.example.com to the
UUCP host darksite.”
This configuration could be set up more simply without the router
adding the suffix .UUCP to darksite only to have the transport take
it off again, but this way is useful because it makes clear the distinction between the
domain name darksite.example.com and the UUCP host
name darksite.

Whenever the router comes across a domain that is in the route file,
it will send the address to the UUCP transport, which subsequently pipes it to the
uux command (described in Chapter 16). If there is a problem,
uux will generate some output and terminate with a
non-zero error code. The setting of
return_fail_output makes sure that the output is
returned to the sender.

If incoming UUCP messages are grouped into files in batched SMTP
format, they can be passed directly to Exim using a command like this:

exim -bS </var/uucp/incoming/001

However, there is one catch. When Exim receives a message locally, it
insists that the sender is the logged-in user that calls it, but for a
UUCP batch we want the senders to be taken from the incoming
messages. Exim will do this if the process that calls it is running as
a trusted user. If you arrange for incoming UUCP
to be handled by a user called uucp, for example, you need to specify:

trusted_users = uucp

in the Exim configuration file to ensure that sender addresses are correctly
handled.